Cephalodium

In an even smaller number of cases – estimated at between 2 and 4 percent of all lichens – species of both a green alga and a cyanobacterium serve as photosynthetic partners.

[5] Most external cephalodia are gall-like structures that are simple in shape: generally rounded, lobed or sac-like.

[6] They can range in size from "minute" to several millimeters across and are typically a different color than the rest of the lichen's thallus.

[7] The fungal partner forms a thick layer of cortical tissue around the outside of each internal or external cephalodium, which helps to reduce the oxygen levels in the structure's interior.

[11] Many species of cyanobacteria grow epiphytically in the environment, where they may come into contact with the upper or lower surface of a lichen.

[13] Lichens containing cyanobacteria (cyanolichens) are among the major contributors of fixed nitrogen to some ecosystems, including mature tropical highland forests.

Illustration showing cephalodium in relation to other parts of the lichen:
1 – Cephalodium
2 – Cyanobacteria
3 – Cortex
4 – Green algal photobiont
5 – Medulla
Small black cephalodia on the upper surface of Peltigera aphthosa